Optical fiber couplers are used in many types of imaging and sensing systems with one or more input fibers and one or several output fibers. Such couplers can be fabricated in different ways, for example by thermally fusing fibers so that their cores get into intimate contact. In many medical imaging systems, illumination is provided at a first port of the coupler via a laser source, sampling of an output signal occurs at a second port of the coupler via a scanning device, and detection can occur at both the first port and a third port of the coupler. Various types of detection are possible, depending on the application.
Medical imaging systems based on optical fiber couplers typically face two major challenges: 1) speckle which is an issue related to temporally and spatially coherent imaging when using lasers and single-mode fibers and 2) a theoretical equipartition limit to the amount of the multi-mode signal that can be extracted from the second port to the third port and injected from the third port to the second port.
There is therefore a need to develop optical fiber couplers that can overcome these challenges, and particularly, a need to develop improved optical fiber couplers for medical imaging and sensing applications.